1. Field of the Invention
The present invention relates to an image forming process comprising a step of developing an electrostatic latent image formed for example in an electrophotographic process or an electrostatic recording process by means of a liquid developer, and also to a novel apparatus adapted for use in such process.
In the prior art of image forming processes such as electrophotography or electrostatic recording there are already known various methods of developing an electrostatic latent image formed on an image carrier such as a photosensitive element or an electrostatic recording material by means of a developer in liquid form (hereinafter referred to as liquid developer). Such liquid developing methods essentially consist of bringing a liquid developer, consisting of toner particles dispersed in an insulating carrier liquid having a volume resistivity in excess of 10.sup.10 .OMEGA..cm and a dielectric constant smaller than 3, for example paraffinic hydrocarbons, into contact with a surface holding an electrostatic latent image thereon to deposit said toner particles onto said latent image on said surface, thereby rendering said latent image visible.
In such liquid developing methods the achievement of a high-speed development has been an important objective in recent years. In order to achieve such high-speed development it becomes necessary to supply a liquid developer of an elevated concentration to a latent image holding surface with a high speed and to perform efficient recovery of excessive liquid developer. It is also important to obtain a developed image of a satisfactory quality from which the liquid developer is sufficiently removed.
2. Description of the Prior Art
As a method approximately satisfying the above-mentioned requirements for the liquid development there is known a method of employing an elastic member capable of holding a liquid therein and provided with a liquid-permeable surface as a supply means for the liquid developer, and conducting the development of electrostatic latent image in a portion of image holding carrier where said elastic member is maintained in pressure contact therewith. Such method is regarded superior to other liquid developing methods for example utilizing a dipping tank of liquid developer or a jet of liquid developer in that the squeeze supply of liquid developer and suction removal of excessive liquid developer can be achieved simultaneously according to the pressure deformation of said elastic member. Said elastic member can be formed in a shape of a roller or of an endless rotary member (hereinafter referred to collectively as elastic roller). As an example of such elastic roller there is disclosed, for example in Japanese Laid-Open Patents Sho No. 52-40336 and Sho No. 52-55644, a developing roller of a structure in which the periphery of a sponge layer is covered with a flexible net member. Such elastic roller is regarded considerably suitable in the application for the development of an electrostatic latent image, as it can be maintained in pressure contact with a rigid member such as a photosensitive member or an insulating member to form a nip of a desired width therewith, squeezing out the liquid contained in said roller or absorbing the liquid thereinto.
For example, in case of an electrophotographic copying apparatus, the development with such elastic roller is conducted in a portion thereof in pressure contact with the surface holding the electrostatic latent image, and in the course of such development said roller is rotated in substantial synchronization with the advancement of said surface, i.e. so that said two members are stationary with respect to each other, in order not to distort or destruct the already formed developed image. It is also usual, while not in development, to interrupt the advancement of said surface and correspondingly the rotation of said roller. However such elastic roller, if left in a state the same as in the course of a developing operation even after the completion thereof, will result in various undesirable phenomena such as (1) gradual plastic deformation of the roller in the contact portion thereof with the image-holding surface with a corresponding loss of elasticity leading to an unsatisfactory development, (2) drying of liquid developer remaining in the vicinity of the contact portion of said roller with said image-holding surface, said liquid developer being solidified on said surface and rendering cleaning of said surface difficult when said surface is to be used again, and (3) gradual evaporation of the carrier liquid of the liquid developer from the roller surface, causing solidification of the liquid developer in the roller and deteriorating the liquid permeability of the roller and thus the developing performance thereof.